1. Field of the Invention
The present invention relates to a sensor attachment position determining method which can suitably be applied to a color image forming apparatus for forming a color image by scanning and exposing a photosensitive body with a light beam and overlaying colors, and an image forming apparatus and an image forming method.
2. Description of the Prior Art
In recent years, tandem-type color printers, copying machines, and composite machines thereof are widely used. In these color image forming apparatuses, scanning optical units, developing units, and photosensitive drums for yellow (Y), magenta (M), cyan (C), and black (BK), an intermediate transfer belt, and a fixing unit are prepared.
For example, in the Y-color scanning optical unit, an electrostatic latent image is formed on the photosensitive drum on the basis of arbitrary image information. The developing unit applies a Y-color toner to the electrostatic latent image formed on the photosensitive drum to form a color toner image. The photosensitive drum transfers the toner image to the intermediate transfer belt. The same process is performed for the remaining colors, i.e., M, C and BK. Color toner images transferred to the intermediate transfer belt are transferred to a paper sheet (transfer paper sheet) and fixed by the fixing unit.
According to a color image forming apparatus of this type, color toner images must be formed on the intermediate transfer belt without any color misregistration in order to transfer the color toner images having no color misregistration to a paper sheet. For this purpose, the positions and tilts of the scanning optical units described above are appropriately adjusted with respect to the photosensitive drums.
The intermediate transfer belt of the color image forming apparatus is subjected to a process called xe2x80x9ccolor registration mark detectionxe2x80x9d periodically or irregularly before formation of a color image based on arbitrary image information. As shown in FIG. 1A, in this detection process, almost V-shaped color registration marks (to also be simply referred to as xe2x80x9cregistration marksxe2x80x9d hereinafter) on an intermediate transfer belt 6 are detected by using reflection-type photosensors (to also be referred to as registration sensors hereinafter) 12A and 12B which are arranged above the two side edge portions of the intermediate transfer belt 6.
As shown in FIG. 1B, the registration sensor 12A comprises a light projecting element SS1 formed from an LED that projects a light beam to the intermediate transfer belt 6, a light receiving element SS2 which receives reflected light from the intermediate transfer belt 6, and lenses SS3. The registration sensor 12B has the same structure as that of the registration sensor 12A.
Light beams emitted from the registration sensors 12A and 12B are shielded by, e.g., registration marks on the intermediate transfer belt 6. In this process, the registration marks are detected by detecting light reflected by the intermediate transfer belt 6.
As shown in FIG. 1A, a positional shift in the sub scanning direction (bow) is detected on the basis of the timing of the detection signal of a line segment P that is parallel to the main scanning direction D1 of the registration marks arrayed in the main scanning direction D1.
In addition, a partial lateral magnification difference xe2x80x9cmxe2x80x9d is detected on the basis of the difference between the timing of detection of the line segments P that are parallel to the main scanning direction D1 of the registration marks and the timing of detection of line segments Q that have a tilt of 45xc2x0 with respect to the main scanning direction D1 of the registration marks. The partial lateral magnification difference xe2x80x9cmxe2x80x9d means a partial variation in magnification in the main scanning direction D1. The partial lateral magnification difference m has adverse effects such as a horizontal image write positional shift on the photosensitive body. On the basis of the pieces of information (detection signals) related to the partial lateral magnification difference xe2x80x9cmxe2x80x9d and positional shift in the sub scanning direction D2, which are detected by the registration sensors 12A and 12B, the position and tilt of each scanning optical unit are controlled to eliminate color misregistration between the respective color images.
According to the conventional xe2x80x9ccolor registration mark detectionxe2x80x9d, the registration sensors 12A and 12B are arranged above the side edge portions of the intermediate transfer belt 6 to detect the partial lateral magnification difference xe2x80x9cmxe2x80x9d and/or positional shift in the sub scanning direction. On the basis of the detection signals, the position and tilt of each scanning optical unit are controlled.
However, the scanning accuracy may vary between the scanning optical units due to the assembly errors of the scanning optical units in the color image forming apparatus or the internal temperature distribution in the color image forming apparatus. Accordingly, at any portion except the side edge portions of the intermediate transfer belt 6 where the registration sensors 12A and 12B are arranged, a considerable difference in partial lateral magnification difference xe2x80x9cmxe2x80x9d and/or positional shift in the sub scanning direction may be generated between the scanning optical units.
If a large difference in partial lateral magnification difference xe2x80x9cmxe2x80x9d and/or color misregistration in the sub scanning direction over the main scanning direction, namely, a scanning line bend is generated between the scanning optical units, the overlay accuracy between Y, M, C and BK colors decreases. This causes conspicuous color misregistration in the color image.
The present invention has been made to solve the above problem in the prior art, and has as its object to provide a sensor attachment position determining method, an image forming apparatus, and an image forming method, which can minimize color misregistration on the entire color image formation surface and also form a high-quality color image with minimum color misregistration.
In order to achieve the above object, according to the first aspect of the present invention, there is provided a sensor attachment position determining method of determining an attachment position of a color misregistration detection sensor of an image forming apparatus which overlays colors on the basis of arbitrary image information to form a color image on an endless-belt-shaped transfer body that is rotatable in a predetermined direction, comprising the steps, which are carried out in a stage of designing the image forming apparatus, of: checking a relationship between the attachment position of the sensor and a formation positional shift of the color image by moving the sensor along an attachment position candidate line that is defined in advance in another direction perpendicular to the rotational direction of the transfer body; finding a specific attachment position on the sensor attachment position candidate line, at which color misregistration of the color image at a side edge portion of the transfer body becomes equal to a maximum value of color misregistration at a portion except the side edge portion; and fixing the sensor at a position that satisfies the found specific attachment position and opposes a color image formation surface of the transfer body.
According to the sensor attachment position determining method, the image forming apparatus can be controlled to correct the color overlay on the basis of a reliable detection signal for which the color misregistration of the color image at the side edge portion of the transfer body becomes equal to the maximum value of color misregistration at a portion except the side edge portion. The color overlay for color image formation can be corrected to eliminate the absolute amount difference of the color misregistration between the central portion in the widthwise direction and the side edge portion of the transfer body.
In order to achieve the above object, according to the second aspect of the present invention, there is provided an image forming apparatus which overlays colors on the basis of arbitrary image information to form a color image, comprising: an endless-belt-shaped transfer body which is rotatable in a predetermined direction; image forming means for overlaying the colors on the basis of the image information to form the color image on the transfer body; color misregistration detection sensor means, arranged to be movable along an attachment position candidate line that is defined in advance in another direction perpendicular to the rotational direction of the transfer body, for detecting color misregistration of the color image formed on the transfer body by the image forming means; and control means for controlling the image forming means to correct color overlay on the basis of a detection signal of the color image by the sensor, wherein the sensor means is fixed at a position that satisfies a specific attachment position at which color misregistration of the color image at a side edge portion of the transfer body becomes equal to a maximum value of color misregistration at a portion except the side edge portion and opposes a color image formation surface of the transfer body.
According to the image forming apparatus, the image forming means for overlaying the colors on the basis of the arbitrary image information to form the color image on the transfer body is controlled to correct the color overlay on the basis of the detection signal of the color image by the color misregistration detection sensor. The sensor menas is attached to a position at which the color misregistration of the color image at the side edge portion of the transfer body becomes equal to the maximum value of color misregistration at a portion except the side edge portion. Hence, color overlay correction can be optimized to eliminate the absolute amount difference of the color misregistration between the central portion in the widthwise direction and the side edge portion of the transfer body. The color misregistration on the entire color image formation surface can be minimized.
In order to achieve the above object, according to the third aspect of the present invention, there is provided an image forming method of causing an image forming system to overlay colors on the basis of arbitrary image information to form a color image on an endless-belt-shaped transfer body that is rotatable in a predetermined direction, comprising the steps, which are carried out in a stage of designing the image forming apparatus, of: checking a relationship between the attachment position of the sensor and a formation positional shift of the color image by moving the sensor along a sensor attachment position candidate line that is defined in advance in another direction perpendicular to the rotational direction of the transfer body; finding a specific attachment position on the sensor attachment position candidate line, at which color misregistration of the color image at a side edge portion of the transfer body becomes equal to a maximum value of color misregistration at a portion except the side edge portion; fixing the sensor at a position that satisfies the found specific attachment position and opposes a color image formation surface of the transfer body; and controlling the image forming system to correct color overlay on the basis of a detection signal of the color image by the sensor.
According to the color image forming method, since the above-described sensor attachment position determining method is applied, the color overlay of the color image can be corrected to eliminate the absolute amount difference of the color misregistration between the central portion and the side edge portion of the rotatable transfer body having an endless belt shape. Hence, the color misregistration on the entire color image formation surface can be minimized.
As is apparent from the above aspects, according to the sensor attachment position determining method of the present invention, the image forming apparatus can be controlled to correct the color overlay on the basis of a reliable detection signal for which the color misregistration of the color image at the side edge portion of the transfer body becomes equal to the maximum value of color misregistration at a portion except the side edge portion. The color overlay of the color image can be corrected to eliminate the absolute amount difference of the color misregistration between the central portion in the widthwise direction and the two side edge portions of the transfer body. Hence, the color misregistration on the entire color image formation surface can be minimized, and the quality of the color image can be increased.
According to the image forming apparatus of the present invention, the color overlay correction can be optimized to eliminate the absolute amount difference of the color misregistration between the central portion and the two side edge portions of the transfer body. The color misregistration on the entire color image formation surface can be minimized. Hence, a high-quality color image with minimum color misregistration can be formed.
According to the color image forming method of the present invention, the color overlay of the color image can be corrected to eliminate the absolute amount difference of the color misregistration between the central portion in the widthwise direction and the two side edge portions of the rotatable transfer body having an endless belt shape. The color misregistration on the entire color image formation surface can be minimized. Hence, the quality of the color image can be increased.
The present invention can be suitably applied to a tandem-type color printer or copying machine, which has an intermediate transfer belt or photosensitive belt, or a composite machine thereof.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples.